The present invention relates to external defibrillators and, in particular, relates to charging and safety devices for automated external defibrillators, as well as control methods for improving the safety, efficiency, and effectiveness of external defibrillators.
Under normal circumstances, the heart functions as a pump to perfuse blood throughout the body. The pump rhythm associated with effective pumping is termed normal sinus rhythm. In certain individuals, the heart ceases to function effectively as a pump and shifts to an ineffective rhythm, termed ventricular fibrillation. Ventricular fibrillation is a non-perfusing pumping rhythm and cannot sustain life. However, it is well known that a high-energy electrical shock delivered to the chest cavity can convert ventricular fibrillation to normal sinus rhythm. This activity is termed defibrillation.
Time is a critical factor in the effectiveness of the administration of defibrillation. Specifically, after the onset of ventricular fibrillation, patient survivability decreases by about ten percent for each minute of delay until the administration of a defibrillation shock.
Emergency first aid and medical personnel are taught to consider a "Chain of Survival" model for emergencies of this type. The chain model has four critical links:
(1) Summoning emergency medical assistance by telephone by dialing "911"; PA1 (2) Cardio pulmonary resuscitation "CPR" to provide oxygen and circulation; PA1 (3) Defibrillation to restore a pumping rhythm; and PA1 (4) Advanced cardiac life support.
Of these four links, merely delaying any one of the associated activities significantly decreases the patient's chances of surviving. Therefore, it is apparent that there is a significant advantage to a patient when a portable defibrillator can promptly arrive on the scene and very shortly thereafter deliver a defibrillation shock to the patient.
Defibrillation has some risks involved. A defibrillation shock can be highly dangerous. Specifically, a person with a normal functioning heart producing normal sinus rhythm has a roughly ten percent chance of having that cardiac rhythm converted to ventricular fibrillation as a result of a defibrillation shock. For this reason, it is important that the defibrillation electrical shock only be administered to those patients truly experiencing ventricular fibrillation and not to patients having been misdiagnosed nor to emergency medical personnel. To distinguish such circumstances, many portable defibrillators are now "automated" which means that they include a diagnostic monitor and analysis system which can gain information about the patient by reviewing the electrical signals the patients heart is producing once the patient's chest has been fitted with defibrillation electrodes. Further, an acceptable range of impedance across the chest is observed as an indication that the electrodes are properly fitted and achieving acceptable contact with the intended patient. Clearly, a loose electrode is a potential hazard to any emergency personnel who might inadvertently receive the electrical shock. This automatic monitoring and analysis stage takes from about 9 to 14 seconds.
For safety reasons, only after an appropriate monitoring signal has been identified, do prior art defibrillators begin to charge their capacitors to generate and temporarily store a defibrillation electrical shock. This takes about 9 to 15 seconds. Thus, once the electrodes are properly fitted upon a patient, from about 20 to 30 seconds must pass before a defibrillation shock can be administered.
It can therefore be well appreciated that there exists a need for better safety arrangements as well as methods of reducing the 20 to 30 second delay between electrode fitting and delivery of a defibrillation shock necessitated by the methodology employed by prior art automated external defibrillators. The present invention, as explain below, addresses the safety and delay problems.